Previous experiences suggested that several palm species could be grown
successfully with minimal winter protection in southwestern Ohio. Here we report
data on an expanded experimental design that included additional palms and
several new experimental plots.

Reprinted with permission from the Vol.
46, No 1, Palms(formerly Principes), Journal of
the International Palm Society

Francko (2000) reported preliminary data on first-year
survivorship and vegetative growth of coldhardy palms in SW Ohio, U.S.A.
Palm survivorship, foliar damage, and subsequent recovery were analyzed
through the 1999 growing season, the winter of 1999–2000, and the 2000
growing season. We also evaluated the efficacy of several published winter
protection strategies and of modified pot-planting technique in reducing
winter damage and mortality.

Materials and Methods

Study Sites. Detailed geographical and climatological information
about the Miami University main campus and off-campus areas in Oxford,
southwestern Ohio were reported in Francko (2000). Winter temperature minima
data from 1989–90 through 1999–00 demonstrate that rural areas around
Oxford, including home garden palm plots (WK sites: forest and near house;
Francko 2000) are mid-Zone 6a microhabitats. Urbanized areas (campus and the
small city of Oxford) are consistently 1.4–2.4ºC (2.5–4.3ºF) warmer and
represent Zone 6b microhabitats. Sheltered areas near large buildings on the
Miami University campus, including the original Hardy Palm Demonstration
Plot (HPDP), are consistently 5.0–6.7ºC (9–12ºF) warmer than rural areas and
effectively Zone 7a to 7a/b microhabitats. In spring 1999 we established
eight additional Zone 7a test plots on the Miami campus, expanded planting
areas at the rural WK site, and integrated small-scale palm plantings into a
church garden (Zone 6b) and a private residential landscape in Oxford (Zone
6a).

Site Preparations and Palms Utilized: Soils in the new plot areas,
in contrast to the original HPDP, are reasonably fertile, circumneutral to
slightly acidic clay-loam and were not amended extensively prior to
installing plants. New planting beds at the WK site which were sited in
compacted heavy clay were amended with compost, humus, and topsoil.

Palms were obtained and planted by the end of April 1999. Each palm
was fertilized two times (early May and mid-July) with a granular,
slow-release (3 month) fertilizer containing micronutrients. We attempted to
provide a combination of rainfall and irrigation water equal to
approximately 2.5 cm per week throughout the 1999 growing season and into
the late fall and winter, but this proved problematic due to extreme drought
and heat conditions.

Pot-Planting and Winter Protection: Tollefson (1999) provided
evidence that pot-planting – setting a containerized palm directly into a
planting hole without removing the container – may reduce root shock
set-back and early mortality in larger palms, and encourages downward root
growth through the lower drain holes of the container during the first
growing season. He suggested that this planting technique could provide
superior results for palms being grown near the limits of their hardiness
range.

To test this view, we employed a modified pot-planting technique in all
containerized palms installed in 1999. Containers were slit ca. 5 cm down
from the top, and the lower drain holes on the sides and bottoms of
containers were carefully slit and expanded prior to placing the container
in the ground. Care was taken not to cut or otherwise damage the root ball.
The top of each container was also trimmed so that it was flush with the
elevation of the root mass. After containers were set into the ground,
planting holes were backfilled with soil, and fertilized and watered through
the 1999 growing season as above.

In mid-December 1999, palms located on the Miami campus and in off-campus
plots were treated with antidessicant spray (Wilt-Pruf), mulched heavily
(ca. 5–8 cm), and crowns and crown cavities were treated with liquid
copperbased fungicide to minimize fungal/bacterial leaf and crown rot. With
the exception of R. hystrix, which has a clumping habit that makes
wrapping difficult, approximately equal numbers of palms of each species
were then either; 1) wrapped with C9 light strings at an approximate density
of one string (50 lights) per meter of plant height, producing roughly 85
watts of heat energy m-3 canopy volume, 2) wrapped with lights and then with
a double layer of lightweight synthetic landscape fabric (ReMay), 3) wrapped
with landscape fabric alone, or 4) left unwrapped as controls.

We used a variation of the third strategy to overwinter Livistona
chinensis and evaluate its potential as a “deciduous” palm for Zone 6
and 7 cultivation. Livistona chinensis specimens at both the HPDP and
at WK ( N = 2 at each site) were allowed to undergo foliar senescence
throughout the fall of 1999. By late December 1999 overnight temperatures
had dropped below -12ºC (10ºF), resulting in leaf necrosis down to the
petiole/spear leaf bases. At this point, dead foliage was cut off, and
plants were covered to ground level with a double layer of landscape fabric
and then a loose covering of bark mulch. Preliminary experiments (Francko
2000) suggested that fabric/mulch coverings thus applied created an interior
environment approximately 13–17ºC (23–31ºF) warmer than ambient air
temperatures, and allowed enough light penetration and air circulation for
palms to retain some green tissue through the winter months. Beginning in
late March 2000, palms were gradually uncovered and shoot/foliar recovery
was monitored throughout the 2000 growing season.

Temperature Measurements: Temperature data were collected in all
plots to determine winter minima and quantify microclimatic variations (Francko
2000). Calibrated thermometers were mounted approximately 20 cm above ground
level on wooden dowel rods. Thermometers with inside/outside probes were
mounted so that the inside probe was within a fabric-wrapped palm or in the
foliage of a plant wrapped with C9 lights. Care was taken to ensure that
temperature probes were kept at least 5 cm away from the nearest light bulb.
Temperatures were recorded approximately 30 min prior to dawn. A minimum of
three thermometers were used at each site, and mean temperature data were
recorded to the nearest degree F.

Quantifying Winter Foliar Damage: Persons attempting to grow palms
in marginal climates require information not only on minimum survival
temperatures but also on the degree of foliar damage to be expected under
defined, sublethal winter conditions. In the temperate palm literature,
foliar damage is usually discussed using qualifiers such as “minor” or
“severe” to describe foliar burn and leaf tissue death (reviewed by Francko,
in press). In this study, we attempted to provide a semi-quantitative,
relative estimate of winter foliar damage for various hardy palm species.

Winter damage in palm leaves often manifests itself in necrotic spotting,
margin burn, and other localized and diffuse tissue damage difficult and
very labor-intensive to quantify with leaf area meters, as well as complete
necrosis of leaf tissue from the leaf tip toward the petiole. Our
experimental palms are genotypically and thus phenotypically variable and
our sample sizes are relatively small, making for inherently noisy foliar
damage data sets. Accordingly, we elected to use a less quantitative but
easier to employ method to assess leaf damage as a function of microclimate
and winter-protection techniques.

Damage assessments of each individual palm were conducted in early April
2000, when all winter damage was easily visualized and before growing season
recovery commenced. A numerical ranking of foliar damage was created by
scoring each plant on the basis of leaf foliage killed (visual observation
of the areal extent of brown and/or necrotic tissue) versus the total foliar
area. The data were grouped into broad numerical rankings: 1 = essentially
no foliar damage, 2 = 15% or less leaf tissue area killed , 3 = 15 to 30%
killed, 4 = 30 to 75% killed, 5 = 75 to 90% killed, 6 = greater than 90%
leaf destruction, but petiole bases green, and 7 = all above ground tissue
killed. Numerical scores for each plant were interpolated to the nearest 0.5
unit. Data for all specimens were pooled by species and mean damage
estimates were computed as a function of microclimate and winter protection.
Individuals that lost spear leaves were also noted.

Microclimatic Variables: The 1999 growing season and subsequent
winter was the second year of a persistent La Nina event that markedly
affected the SW Ohio climate. Extreme summer and fall heat stressed newly
planted palms. Mean high temperatures for summer and fall were several
degrees C above historical means, and temperatures reached or exceeded
37.8ºC (100ºF) in Oxford, Ohio on three occasions. A persistent drought
occurred from mid-June through October 1999; during a six-week period in
mid-summer no rainfall occurred. As noted in the Methods section, we
attempted to provide approximately 2.5 cm of irrigation water per week to
each experimental plot during the growing season, and with rare exceptions
this goal was met.

Nonetheless, most palms were planted in full sun habitats or in shadier
plots where root competition for moisture from established vegetation was
extreme. Nearly all of our palms suffered visible drought stress during
summer and early-mid fall 1999 (folding leaves, yellowing and premature leaf
senescence) and entered the cooler months of fall and the winter season in
less than ideal condition.

This was especially true of 3-gallon-sized specimens of Trachycarpus
takil (N = 11), which were just beginning to develop trunks. Several
specimens lost their spear leaves during the summer and in all specimens
approximately 30 to 70% of extant foliage was yellowed by October 1999.
However, unless leaves were totally senesced prior to winter they were
included as “live” tissue for purposes of winter foliar damage estimation
the following spring.

Winter conditions in Oxford, Ohio during 1999–2000 were similar to those
reported for 1998–99 (also a La Nina year; Francko 2000); relatively mild
overall, but including a prolonged, extreme cold spell in January. From 16
to 31 Jan 2000, air temperatures in the coldest Zone 6a palm plots (WK site)
remained below 0ºC for all but a few hours. Beginning on 21 January,
overnight low temperatures at the WK Forest site reached - 18ºC (0ºF) or
lower for eight consecutive nights, the longest sub-zero F event since at
least 1983–84. The winter minimum of -24ºC (-12ºF) on 27 January 2000 was
considerably lower than the average for the 1990s (-21.6ºC/-6.8ºF). As in
previous years (Francko 2000), Miami University campus plots represented
much warmer microclimatic regimes, consistently 5º to 6ºC (9º to 12ºF)
warmer than the WK site on the coldest nights.

3 (top). Relative growth of first year Trachycarpus
fortunei. Specimen in front of meter stick was approximately the same
size as the containerized individual in the foreground when installed in
spring 2000.

4 (bottom). Regrowth of Livistona chinensis (early
Oct 2000) after being killed back almost to ground level during winter
1999–2000. Keys on ground in front of plant illustrate scale.

Effect of C9 lights and Fabric Wrapping on Cold Exposure: Air
temperatures near the leaves of palms wrapped with landscape fabric and C9
lights, either singly or together, were significantly higher than those of
unwrapped control palms, at both campus and WK sites. Fabric wrapping alone
yielded inside-wrap temperatures that were 3.3–6.7ºC (6–12ºF) warmer than
temperatures outside the wrapping; lower temperature gradients were noted
under windy conditions. For wrapped palms in the HPDP and other campus
sites, the minimum temperature to which foliage was exposed to was -15.0ºC
(5ºF) compared with -19.4ºC (-3ºF) in unwrapped control plants. Livistona
chinensis plants covered to ground level as described earlier were
approximately 14ºC (25ºF) warmer than ambient throughout the duration of
sub-zero F cold.

Palm foliar canopies wrapped with C9 lights alone were approximately
1.1–2.2ºC (2–4ºF) warmer than unwrapped plants in calm air, but thermal
enhancement was nil when sustained wind speeds exceeded a few km per hour.
Not surprisingly, C9 lights coupled with an outer landscape fabric wrap
produced the largest thermal gradient, ranging from 9.4–17.8ºC (17–32ºF)
above ambient inside the wraps compared with unwrapped control palms. In the
coldest WK sites, temperatures never dipped below -9.4ºC (15ºF) when lights
were on, even when the outside air temperature dipped to -24ºC (-12ºF).

Palm Survivorship, Damage, and Recovery: Despite the warmer
temperatures produced by C9 lights and fabric wraps, foliar damage
assessment data in spring 2000 failed to produce any statistically
significant differences (P < 0.05; paired sample t-tests) between wrapped
palms and unwrapped controls, for any of the treatment variations employed
or for any taxa. Accordingly, Table 1 presents foliar damage indices for
pooled samples of all treatments of each species collated by microclimate.
Although there was less damage overall noted in plants sited in Zone 7a
microclimates, these differences were statistically significant (P < 0.05;
paired sample t-tests) only in Rhapidophyllum hystrix.

The relative degree of leaf tissue damage we observed in palm species
closely paralleled the consensus minimum survival temperatures for these
species recorded in the literature (SEPEPS 1994, Walters 1998, Noblick 1998,
Avent 2000, McKiness 2000, Francko 2000). Specimens of R. hystrix,
generally recognized as the most cold-hardy palm species in terms of
survival, were virtually undamaged by winter conditions on the Zone
7a-microclimate Miami campus, and approximately 1/3 of the foliage was
winter burned on plants exposed to colder temperatures at WK sites. Sabal
minor and S. minor “Louisiana” foliage was slightly more
sensitive to cold than R. hystrix at both sites. Both Trachycarpus
fortunei and T. takil were damaged to similar extents, with
approximately 75 to 80% defoliation noted at both the campus and WK sites.
Several plants lost their spear leaves. Although our sample size (N = 3) was
too small for statistical comparisons, T. fortunei sited in campus
plots that never received direct afternoon sun were less damaged
(approximately 25% foliar burn) than plants sited in full sun locations.
Sabal palmetto, S. etonia, and S. bermudana specimens
sited in fun sun campus plots were almost completely defoliated, even when
wrapped with landscape fabric, although one C9 light/fabric wrapped S.
palmetto in the HPDP retained perhaps 25% of its green foliage through
the winter. A single S. etonia growing in the same campus “shade”
plot as T. fortunei above retained significant green foliage, even
though it served as an unwrapped control specimen. The single S. etonia
grown at the WK site was defoliated and lost its spear and all green
above ground tissue, despite the observation above that temperatures inside
the foliar wraps did not drop below -9.4ºC (15ºF). Damage to Serenoa
repens on campus was similar to that noted in Trachycarpus
species, and the sole specimen of S. repens planted at WK was
defoliated but retained green petiole bases.

Recovery During 2000 Growing Season: The 2000 growing season, in
contrast to 1998 and 1999, produced near-ideal conditions for plant growth.
Rainfall was slightly above average, mean high temperatures in summer were a
few degrees C cooler than average, and the longest period of summer drought
was approximately two weeks. Despite sometimes major foliar damage and, in
some cases loss of spear leaves, all but seven of the plants survived and
recovered in spring-summer 2000 (Table 1, Figs. 1 and 2). Palms began
producing new leaves in mid-April 2000, and at that time we pruned damaged
leaves to remove dead tissue. A few Trachycarpus specimens that
survived the winter with fairly intact foliage lost their spear leaves well
after the onset of warm weather. We suspected that this was caused by a
fungal/bacterial infection in the crown cavity, and after a 2-week treatment
with copper-based fungicide, these plants began to develop a new spear and
subsequently recovered completely.

Although plant aspect and form are somewhat subjective criteria,
sufficient regrowth had occurred by late May 2000 that palms at both the
campus and WK sites looked normal and healthy to the casual observer. By
late summer all surviving individuals of R. hystrix, S. minor
(both varieties), T. fortunei and T. takil had visibly grown
larger than they were at the end of the 1999 growing season (Fig. 3). In
general R. hystrix, S. minor varieties, and T. takil
produced three or four fully expanded leaves and an expanding spear by early
October 2000. T. fortunei specimens grew four to six new leaves and a
spear and added or developed 5 to 10 cm of new trunk.

Sabal palmetto, S. etonia and S. bermudana specimens
on campus also added three to four new leaves and a spear during the summer
and by the end of the growing season were approximately the same size or a
bit larger that they were at the end of 1999. The containerized, completely
defoliated, and spearless S. etonia specimen growing at the WK site
was excavated in April 2000. Dead tissue was removed and the crown cavity
was sprayed with copper-based fungicide. A nascent spear leaf began to grow
from this plant by early May. The palm was removed from its container and
replanted in a different WK location. By the end of the 2000 growing season
this palm was approximately a meter tall, with three fully expanded leaves
and an expanding spear, although the first leaf to emerge in the spring
remained severely stunted.

Serenoa repens and Livistona chinensis specimens at both
sites produced two or three new expanded leaves and an expanding spear per
trunk during the 2000 growing season. In both species, plants grew to
approximately 50% and 75% of the overall size they were at the end of the
1999 growing season, respectively. Livistona chinensis is seldom
grown outdoors in areas colder than USDA Zone 8b (SEPEPS 1994, Riffle 1999).
Our specimens were typical, greenhouse-grown ‘tropical’ plants with
characteristically lush foliage. The observation that properly overwintered
L. chinenis could recover from near complete defoliation to produce
50 cm-tall plants with a crown spread exceeding one meter (Fig. 4) suggests
that this species and perhaps similar palms may have utility in temperate
gardens as deciduous understory specimen plants.

Efficacy of Pot Planting: As noted above, pot-planted palms were
severely drought stressed entering the winter season. Although C9 lights and
fabric wraps reduced the level of cold exposure in our palms, enhanced
thermal regimes did not translate into reduced foliar damage. It is possible
that these two observations are interrelated and consistent with the view
that our decision to pot-plant 1999 specimens adversely affected their
viability during the 1999 growing season and into the winter season.

By expanding the lower drain holes and partially slitting the sides of
containers we hoped to facilitate root growth into the surrounding soil. We
also thought that this strategy would permit at least some capillary flow of
water from surrounding soils into the containerized root mass. Some palm
species, most notably L. chinensis, R. hystrix and S. minor,
rooted rapidly through the lower drain holes and slit sides and were solidly
anchored to the soil within 4 to 6 weeks after planting. Other species,
including T. fortunei, T. takil, S. palmetto, S.
etonia and S. bermudana, were clearly not well rooted even by the
end of the 1999 growing season.

In spring 2000, we excavated dead specimens of Trachycarpus as the
severely damaged WK specimen of S. palmetto described earlier. Root
growth outside of the containers was nonexistent in every plant. These
plants could access soil moisture only from within their containers and had
not developed roots that extended below the soil freeze line during the
mid-January 2000 cold spell. Under such conditions, it was not surprising
that summer drought stress and winter foliar injury or mortality occurred in
many of our palms, even those protected by foliar wraps and C9 lights. With
a frozen root zone, the warming effects of C9 lights and foliar wraps might
actually have caused more harm than good, due to enhanced photosynthetic
water demand in the relatively warm and well-lit leaves. Taken together, our
data do not support efficacy of pot-planting in enhancing first-year palm
survivorship or reducing damage, at least under the rigorous environmental
conditions that characterized our 1999–2000 experimental season.

Additional Considerations: Although our data did not support the
hypotheses that pot-planting or artificial heating and wrapping could
significantly reduce winter injury and mortality in first-year palms, some
additional considerations are necessary. In contrast to pilot-year data
reported in Francko (2000), where palms were almost completely covered by
drifted snow during the coldest periods of the winter, palms described here
were covered only with a few cm of snow during the severe January freeze. In
addition, the duration of the extreme cold event was much longer than that
of 1998-99. Palm foliage was thus almost completely exposed for more than
one week to the full effects of extremely cold air, very low wind chills,
and in most cases full winter sun. Under these conditions C9 light wrapping
may have been counterproductive in that they melted the snow that might have
provided at least partial foliar and trunk insulation from sun and damaging
winds. Strings of lights can provide a heat boost of several degrees C, thus
protecting marginal palms under the short-duration, relatively minor freeze
events characteristic of USDA Zone 7b and warmer locales. They may be much
less effective in protecting newly planted palms under the more extreme and
longer duration cold events in Zone 6.

It is also critical to note that our dataset deals solely with palms that
have not been in the ground long enough to become well established and
develop deep and vigorous root systems. After 3 to 4 growing seasons a
well-established palm should possess a root zone that extends well below the
typical soil freeze depth in winter, even in Zone 6 sites, and such plants
would likely benefit from foliar wraps and other active-protection
strategies.

Acknowledgments

This work was supported the Ohio Plant Biotechnology Consortium and by
the Department of Botany and Campus Services Office of Miami University. We
thank Gerry McKiness for his willingness to find and provide outstanding
specimen palms.